This 100mm Macro belongs to a very peculiar family of Sigma's YS-lenses. Judging them by sharpness and such make no justice to them as their fanciness is more in intriguing versatility of fystem focusing for example. Indeed, there are two focusing systems present: First is the big focusing ring - a typical focusing where it can be focused from ~0.85m to inf. Second is the "system focusing" found from the front of the lens. It's for macro shooting up to 1:1 reproduction. What Sigma doesn't tell is that it can be used also to soft focusing, as it adds soft glow to an image when used. The more you turn it, the more softness you get. By using aperture and these two focusing systems, it's possible to get photos from sharp to very soft with huge glow and creamy background.
If you like extraordinary lenses, I recommend this (and other Sigma YS-lenses) warmly.